1. Field of the Invention
This invention relates to seismic exploration and more specifically, to marine seismic survey systems.
2. Related Art
Marine seismic exploration investigates and maps the structure and character of subsurface geological formations underlying a body of water. For large survey areas, seismic vessels tow one or more seismic sources and multiple seismic streamer cables through the water. The seismic sources typically comprise compressed air guns for generating acoustic pulses in the water. The energy from these pulses propagates downwardly into the geological formations and is reflected upwardly from the interfaces between subsurface geological formations. The reflected energy is sensed with hydrophones attached to the seismic streamers, and data representing such energy is recorded and processed to provide information about the underlying geological features.
Three-dimensional (3-D) seismic surveys of a grid provide more information regarding the subsurface formations than two-dimensional seismic surveys. 3-D surveys may be conducted with up to twelve or more streamers that form an array covering a large area behind the vessel. The streamers typically vary in length between three and twelve kilometers. Tail buoys attached at the streamer distal ends may carry radar reflectors, navigation equipment, and acoustic transponders. Hydrophones are positioned along each streamer. The hydrophones may or may not be wired together in receiver groups spaced along each streamer. The in-line interval between hydrophones or groups of hydrophones ranges between about 3 and 25 meters, with 12.5 meters comprising typical interval spacing.
Since the grid is often much wider than the array, the tow vessel must turn around and tow the array in laps across the grid, being careful not to overlap or leave large gaps between the laps across the grid.
A multiple streamer array requires diverters near the vessel to pull the streamers outwardly from the direct path behind the seismic tow vessel and to maintain the transverse or crossline spacing between individual streamers. Diverters rely on hydrodynamic lift created by forward motion through the water to pull the streamers outwardly and to maintain the transverse position relative to the vessel path. If forward motion changes due to ocean currents and other environmental factors, the diverters may not maintain the desired streamer position.
In 4-D geophysical imaging, a 3-D seismic survey is repeated over a grid that has been previously surveyed. This series of surveys taken at different times may show changes to the geophysical image over time caused, for example, by extraction of oil and gas from a deposit.
It is important that the sources being used to generate the acoustical pulses be located as closely as possible to the same location as in previous surveys over the same grid. This, of course, has been difficult to accomplish in a marine survey because the acoustical source arrays are typically towed behind the tow vessel and are subject to wave and current movement.
In addition to the deployment and operation difficulties associated with towing multiple streamers, conventional techniques limit the ability to position source equipment and receivers in different relative positions and orientations. Because the sources and receivers are towed behind the same seismic vessel, array design is limited by the tow configuration and vessel layout. Each towed array is also subject to crosscurrents, wind, waves, shallow water, navigation obstacles, and steering limitations that limit the coverage provided by the survey system.
Conventionally, attempts to control the location of source arrays have included attaching the arrays to distance cables running to other deflectors or tow cables. These attempts have not provided optimal control of the location of the source arrays under towing conditions. The deflector wing inventions disclosed in WO2004092771 A2, published Oct. 28, 2004, (the '771 application) address these problems for the case of using a single source array. By attaching one or more deflector wings to the front of the source array, or a winch to the front of the source array that acts on a lateral deflector, the source array location may be controlled. However, additional problems present themselves when so-called dual sources are utilized. A dual source arrangement comprises two or more source arrays being positioned symmetrically about the tow centerline and configured to fire in an alternating manner. In this configuration each source array needs to be deflected laterally to its nominal position, different from zero. If one in addition desires to steer each source array so as to correct for deviations from the nominal position and/or steer the source arrays to follow a path from a previous survey, the source deflectors need both to spread the source out to a nominal position and to be able to correct for the deviations on top of the nominal positions. To be able to do this the source deflectors need to be of a considerable size, larger than what is required for deflecting in order to counteract for the deviations from the nominal position only, thus adding to the cost of the system, as well as complicating the handling during deployment and retrieval of the source system.
Accordingly, a need exists for improved techniques and equipment for conducting marine seismic operations. It would be advantageous if such techniques and equipment could be utilized with both single and dual acoustic sources, especially while positioning them in desired locations while being towed behind a vessel.